We previously demonstrated that high density lipoproteins (HDL) bind T4 in human plasma through a single binding site on apoA-I and that low density lipoproteins (LDL) bind T4 through 3 binding sites on apoB-100. By photoaffinity labeling with [(125)I]T4 and its inhibition by monoclonal antibodies to the apolipoproteins, we have investigated the location of the T4 binding sites. In apoA-I, the T4 site is in the N-terminal third of the molecule. In apoB-100, the T-4 sites are outside the LDL receptor binding domain and are located in the molecular domains designated as apoB-26, apoB-44 and apoB-30. Conformational changes due to interaction with lipids appear to result in altered patterns of monoclonal antibody inhibition.